a. Field of the Invention
This invention relates to the field of optics and, more particularly, to improvements in instrumentation for use in that field. Still more specifically, this invention pertains to instrumentation adapted for use with what will be referred to as "optical-type" radiations (i.e., infrared, visible and ultraviolet radiations of wavelengths conforming to the laws of optics relating to transmission, reflection and refraction) and concomitantly provides, first, an improved kind of optical masking device having masking characteristics (in terms of transmissivity versus reflectivity and/or opacity) that are selectively and quickly alterable under electrical control while the masking component and all parts of the latter remain fixed in a stationary position, and, secondly, improved optical apparatus employing such masking devices as components thereof for a variety of possible applications. One exemplary application for the invention, for which there is an immediate and substantial need, and with respect to which the invention is hereinafter primarily disclosed for illustration, is in connection with computerized, infrared spectroscopic systems utilizing Hadamard transforms or analogous mathematical techniques for spectral analysis.
b. General Background Prior Art
Conventional devices employed as components in instrumentation for manipulating, analyzing or responding to optical-type radiations include planar mirrors for changing the direction of travel of such radiations, curved mirrors for both collimating or focusing and changing the direction of travel of such radiations, partially "silvered" mirrors for splitting a beam of such radiations into a pair of beams travelling in different directions, lenses for collimating or focusing such radiations, prisms for separating and dispersing such radiations into components corresponding to the wavelengths present in the radiations with the direction of travel of each component being differently displaced, reflective or transmissive diffraction gratings for the same general purpose as prisms and also changing the general direction of travel of the dispersed components when the dispersive element is reflective, various kinds of "photoelectric" sensors for detecting such radiations and responding to the intensity thereof either by producing a corresponding electrical output or a corresponding change in the value of an internal electrical impedance of the sensor, various combinations of the foregoing, etc. Although it is to be understood that apparatus embodying this invention may appropriately employ any of such conventional optical devices as system components, no claim is herein made to any of such components per se.
Another type of device conventionally employed in various types of apparatus for use in connection with optical-type radiations is commonly referred to as a "mask". The purpose of such masks is to permit the passage, by transmission or reflection, of one or more selected cross-sectional portions of such radiations (or one or more mutually displaced wavelength components thereof), while blocking the passage of other portions (or components) of such radiations. A typical mask of the transmission type utilizes one or more apertures or transparent zones in an otherwise opaque plate or the like, and a typical mask of the reflective type utilizes one or more mirrored or reflective zones upon an otherwise transmissive (or opaque and relatively non-reflective) plate or the like. A simple example is the beam restricting "entrance mask" having a single slit in an opaque plate, as commonly employed in many spectrometers. It is also known practice to utilize more than one masking component in succession along the path of travel of optical-type radiations (for example, successive masking components each having a respectively perpendicular, elongate, rectangular aperture or reflective zone may be employed to provide passage for a square cross-sectional portion of such radiations.) More recently, the advent of using mathematical techniques such as Hadamard transforms in spectroscopic analysis systems has brought into common usage a type of mask having a plurality of transmissive or reflective zones (typically a number of parallel, rectangular zones spaced from each other in some predetermined arrangement in which they occupy only a portion of the overall area of the mask) for passing a corresponding group of mutually displaced wavelength components of optical-type radiations, the particular group of components passed being dependent upon the precise positioning of such a mask relative to the paths of the radiation components. Again, apparatus embodying this invention may appropriately utilize the foregoing types of conventional masks (for example, as an entrance mask), but no claim is made to any of those general types of masks per se (i.e., apart from the construction thereof with respect to permitting the masking zone pattern to be "altered" under electrical control).
c. Hadamard Transform Mathematical Technique of Analysis
In very general terms, the Hadamard transform mathematical technique of analysis is applicable to situations involving a plurality of distinguishable quantities, each of which may exist in some unknown magnitude or may be absent, in which it is desired to accurately determine the existence and magnitude of each quantity which exists under circumstances rendering it more feasible or expedient to accurately determine the aggregate magnitude of various subset groups of such quantities than to determine the existence and magnitude of each quantity individually (for example, when the number of quantities is very large). In such situations, the Hadamard technique (and analogous variations thereof) essentially employs matrix transforms to solve a set of simultaneous equations in which the coefficients for each equation are based upon measurements of the aggregate magnitude of a different subset group of the possible quantities. As applied to spectroscopic analysis of the presence and intensity (or amount of energy) of "spectral element" components of different wavelengths (typically, relatively narrow spectral bands of radiation each constituting a small interval of the wavelength spectrum) in optical-type radiations from a source of the latter, wherein the ability to separate and accurately measure the intensities of individual wavelength components directly is inherently limited by the resolution capabilities of the wavelength component dispersing element, the sensitivity of the detecting and measuring element and other factors, the Hadamard technique permits data derived from a plurality of measurements of the aggregate intensity of differing subset groups of the possible wavelength components which may be present to provide accurate results concerning the presence and intensity or absence of each wavelength component within a spectrum range of interest. The Hadamard technique or/and its application to spectroscopic analysis, including the nature and number of mask patterns needed for analysis of a given spectral range and further details of the involved computations, are more fully and formally discussed in the literature; for example, see: the chapter entitled "Hadamard Transform Spectroscopy" by W. G. Fateley, et al. at pages 89-118 of the book "Analytical Applications of FT-IR to Molecular and Biological Systems" edited by J. R. Durig and published by D. Reidel Publ. Co. in 1980, the book "Hadamard Transform Optics" by Martin Harwit, et al. published by Academic Press, Inc. in 1979, the chapter entitled "Hadamard Transform Analytical Systems" by Martin Harwit at pages 173-197 of the book "Transform Techniques in Chemistry" edited by P. R. Griffiths and published by Plenum Press in 1978, the article entitled "Fourier and Hadamard Transform Methods in Spectroscopy" by A. G. Marshall, et al. at pages 491A-504A of the journal "Analytical Chemistry", Vol. 47, No. 4, April 1975, the article entitled "Hadamard-Transform Image Scanning" by J. A. Decker, Jr. at pages 1392-1395 of the journal "Applied Optics", Vol. 9, No. 6, June 1970, and the article entitled "Hadamard Transform Image Coding" by W. K. Pratt, et al. at pages 58-68 of the journal "Proceedings of the IEEE", Vol. 57, No. 1, Jan. 1969. Recent U.S. Patents relating to the use of Hadamard transform techniques, which discuss the type of computations involved or specific apparatus for making the same, although relating primarily to the image recognition field or to the computational apparatus itself, include Despois, et al. U.S. Pat. No. 4,389,673, Lux No. 4,134,134, Joynson, et al. U.S. Pat. No. 3,982,227, McGlaughlin U.S. Pat. No. No. 3,969,699, Radcliffe U.S. Pat. No. 3,859,515 and Muenchhausen U.S. Pat. No. 3,815,090. The algorithmic and computational aspects of employing Hadamard transform techniques in various applications are now well known and are not per see claimed herein. It is also recognized that the use of appropriately programmed electronic computers is now generally regarded as the most convenient and preferred method of performing the computations involved in the Hadamard technique.
d. Prior Masking Devices
As previously noted, made clear in the mentioned literature and also indicated by the Decker U.S. Pat. No. 3,578,980, the conventional and commonly accepted form of optical masking devices has long involved plate-like elements having one or more fixed transmissive or reflective zones. Such masking devices are quite satisfactory in applications in which the masking configuration need not be altered. However, in applications in which it is essential that the masking configuration be altered (such as in spectroscopy employing Hadamard transform or analogous techniques), it has heretofore been necessary either to successively substitute differently configured masking components or to provide mechanical means for readjusting the location of a single masking component, in both cases giving due attention to ensuring that the substituted or shifted mask is relocated with the utmost precision. These considerations and the resultant high cost of both equipment and time required for utilization, as well as the possibly deleterious effect upon accuracy of any imprecision of manual emplacement or mechanical adjustment of the masking component(s), has stood as a significant impediment to the construction and use of practical spectrometers and other apparatus for dealing with optical-type radiations in a manner to realize the acknowledged potential benefits of Hadamard transforms or analogous mathematical techniques of measurement and analysis.
With regard to previous device, which may be of some background interest in relation to the specific nature and construction of the improved masking device provided by this invention, the Wajda U.S. Pat. No. 4,007,989 recognizes the existence of the same problems arising from movable masking components as addressed by this invention and discloses a "filter" for use in Hadamard transform spectrometers that has no moving parts, but employs an element provided with multiple "fly's-eye" lenses for respectively focusing radiation components of differing wavelengths upon corresponding ones of an associated array of photodiode detectors in conjunction with electrically switched scanning of the electrical outputs from the detectors. Certain ones of the lenses in the Wajda device are "rendered opaque" in an unspecified but apparently fixed manner to present a Hadamard technique compatible pattern. However, although the overall device is referred to as a "Hadamard mask", only a single, unalterable pattern of optical radiation masking is provided, and appropriate electrical scanning of the multiple detectors is relied upon for implementing a Hadamard transform technique. No suggestion is found in the Wajda disclosure of an alterable mask for optical-type radiations or how such a device might be provided.
Other prior U.S. Patents of possible background interest are the Torok U.S. Pat. No. 3,861,784, which employs magnetic stripe domain technology to provide electrically controllable equivalents of a diffraction grating, a Fresnel lens or the like, and the Fleisher U.S. Pat. No. 3,402,001, which provides a Fresnel lens equivalent for monochromatic, polarized light from a laser by means of an electric potential applied between concentric, annular electrodes on opposite sides of a plate of material adapted to have its optical transmissive properties polarized by the electrical field applied across its thickness, and the Buhrer U.S. Pat. No. 3,813,142, which also provides a diffraction grating equivalent by applying an electrical field between electrodes associated with an intervening film of material whose optical index of refraction is changed by the field.
Since the mask provided by this invention employs a film of diachromic crystalline or polycrystalline material, such as vanadium dioxide, it should also be noted that a number of researchers have investigated and reported upon the inherent chemical, crystalline, optical, electrical and other physical properties of both vanadium dioxide and thermodiachromic or electrodiachromic optical, effects when the material is stimulated by heat or the flow of electrical current therethrough to traverse its semiconductor-metal transition level (or the similar effects exhibited by some organometallic complex compounds). Although no prior suggestion of the application of such properties of such materials for implementing alterable masking devices of the kind provided by this invention is known, the information provided by such research reports concerning specific parameters of particular properties may be useful to persons following this invention in selecting among available materials and otherwise designing masking devices in accordance with this invention which will be optimized for particular wavelength regions of the opticaltype radiation spectrum or for specialized applications or environments. Accordingly, the following papers ae noted and identified: "Infrared Optical Properties of VO.sub.2 Above and Below the Transition Temperature", Barket et al., Phys. Rev. Lett., Vol. 17, No. 26; "Electronic Properties of VO.sub.2 Near the Semiconductor-Metal Transition", Berglund et al., Physical Rev., Vol. 185, No. 3; "High-Speed Solid-State Thermal Switches Based on Vanadium Dioxide", Cope et al., Brit. J. Appl. Phys., 1968, Vol. 1, Sec. 2; "Filamentary Conduction in VO.sub.2 Coplanar Thin-Film Device", Duchene et al., Appl. Phys. Lett., Vol. 19, No. 4; "Optical Properties of VO.sub.2 Between 0.25 and 5eV", Verleur et al., Phys. Rev., Vol. 172, No. 3; "Optical Storage in VO.sub.2 Films", Smith et al., Appl. Phys. Lett., Vol. 23, No. 8; "Semiconductor-to-Metal Transitions in Transition-Metal Compounds", Adler et al., Phys. Rev., Vol. 155, No. 3; "Two Switching Devices Utilizing VO.sub.2 ", Walden et al., "IEEE Transactions on Electron Devices", Vol. ED-17, No. 8; "Change in the Optical Properties of Vanadium Dioxide at the Semiconductor-Metal Phase Transition", Mokerov et al., Sov. Phys. Solid State, Vol. 18, No. 7; "Features of the Optical Properties of Vanadium Dioxide Films Near the Semiconductor-Metal Phase Transition", Gerbshtein et al., Sov. Phys. Solid State, Vol. 18, No. 2; "Influence of Stoichemistry on the Metal-Semiconductor Transition in Vanadium Dioxide", Griffiths et al., J. Appl. Phys., Vol. 45, No. 5; and "Semiconductor-to-Metal Transition in V.sub.2 O.sub.3 ", Phys. Rev., Mar. 15, 1967. With regard to methods for making thin films of vanadium dioxide, also see: "Reactivity Sputtered Vanadium Dioxide Thin Films", Fuls, et al., Appl. Phys. Lett., Vol. 10, No. 7; and "Preparation of VO.sub.2 Thin Film and its Direct Optical Bit Recording Characteristics", Fukuma et al., Appl. Optics, Vol. 22, No. 2.